Various catalysts which substitute fluorine atoms for chlorine atoms have been proposed for use in gaseous phase reactions. Frequently, these catalysts are oxides or halides of chromium, aluminum, cobalt, iron, titanium, nickel, copper, palladium or zirconium; which may be used as they are or on various supports.
French Patent No. 720,474 and its Certificate of Addition No. 43,972 teach gaseous phase fluorination of hydrocarbons containing a halogen other than fluorine by metallic halide catalysts.
U.S. Pat. No. 2,210,369 discloses the fluorination of C.sub.1 to C.sub.3 halohydrocarbons over catalysts having a chromium halide base deposited on coke or active carbon.
French Patent No. 2,000,688 teaches the use of a chromium trifluoride catalyst supported on wood charcoal, petroleum coke, or coal carbon in reactions of chlorine and hydrofluoric acid with tetrachloroethylene.
British Patent No. 896,068 and U.S. Pat. No. 3,157,707 describe catalysts comprising a chromium oxide base deposited on activated alumina. The catalysts are useful in the preparation of fluorinated compounds such as trichlorotrifluoroethane and dichlorotetrafluoroethane from hexachloroethane.
U.S. Pat. No. 3,992,325 discloses the use of a chromium .gamma.-CrOOH hydroxide-oxide catalysts which is deposited on mineral fluorides as, for example, alkaline-earth fluorides.
U.S. Pat. No. 2,775,313 describes the use of aluminum trifluoride, prepared from gaseous or liquid aluminum trichloride in the fluorination of hexachloroethane formed in situ.
These commonly used catalysts are basically suitable for gaseous phase fluorination of chloroalkanes or chlorofluoroalkanes in fixed bed reactor systems. In fluidized bed reactors, which require regular-shaped particles and homogeneous granulometry, the prior art catalysts are inadequate and inefficient for use in fluorination processes. Simple grinding of the catalysts, followed by sifting for the selection of suitable-sized particles provides irregularly-shaped grains which are not suitable for use in fluidized bed reactors. Consequently, their use leads to a significant loss of the catalyst, which necessitates recharging the reactor at various intervals during the process.
The prior art catalysts often demonstrate at least one of the following disadvantages:
low rate of conversion of hydrofluoric acid PA1 low productivity PA1 low selectivity PA1 high amounts of asymmetric isomers in the production of trichlorotrifluoroethane and dichlorotetrafluoroethane PA1 (a) adding ammonia to an aqueous solution of an aluminum salt and phosphoric acid; PA1 (b) washing the precipitated aluminum phosphate; PA1 (c) forming the washed aluminum phosphate into pellets by granulation-extrusion; PA1 (d) impregnating the pellets with an aqueous solution of chromium trioxide; PA1 (e) reducing the chromium trioxide with an alcohol; PA1 (f) drying the product with, advantageously, heated air at a temperature of about 150.degree. C.
Known fluorination catalysts, especially those containing aluminum, catalyze dismutation reactions such as: EQU 2CF.sub.2 Cl--CFCl.sub.2 .fwdarw.CF.sub.2 Cl--CCl.sub.3 +CF.sub.3 --CFCl.sub.2
or isomerization reactions such as: EQU CF.sub.2 Cl--CFCl.sub.2 .fwdarw.CF.sub.3 --CCl.sub.3
Asymmetric isomers of trichlorotrifluoroethane (CF.sub.3 --CCl.sub.3) and dichlorotetrafluoroethane (CFCl.sub.2 --CF.sub.3) are undesirable in many applications since they are more reactive, and consequently, more unstable than the symmetric derivatives (CCF.sub.2 Cl--CFCl.sub.2 and CClF.sub.2 --CClF.sub.2.)